The present invention relates to an apparatus and method involving selective heating of the edge of a container sidewall prior to a necking operation and, in particular, heating substantially to an annealing temperature.
A number of container fabrication processes involve closing one or more open ends of a container body with a container endpiece which has a perimeter different from the sidewall perimeter. For example, a typical 12 ounce aluminum beverage container has a cylindrical side wall with a diameter of roughly 2xc2xd inches (about 6.4 cm) but an upper endcap with a diameter of roughly 2xe2x85x9 inches (about 5.4 cm). Thus, in many container fabrication processes, the region adjacent the sidewall open end(s) is faired in (or, in some cases, faired out) to match the diameter of the sidewall edge to the diameter of the edge of the endcap (typically allowing for provision of a rim or flange for double seaming or otherwise connecting the two pieces). This process of reshaping the cylindrical sidewall to a portion of the cylindrical sidewall to reduce (or enlarge) the diameter near an open end is referred to as xe2x80x9cneckingxe2x80x9d. Many of the pre-necking processes are performed at various temperatures, including somewhat elevated temperatures such as elevated temperatures employed in washing, drying of inks or other coatings, and the like. Typically, such upstream processes are performed at temperatures substantially below the annealing point of the container material such as below about 500xc2x0 F. For example, for the aluminum alloys used in many beverage containers, the annealing temperatures are in the range of about 500xc2x0 to about 825xc2x0 F., preferably around 600-700xc2x0 F. and more preferably around 625xc2x0 F. or higher. A number of procedures and apparatuses to achieve necking are used, or have been proposed, including die necking, spin necking, and the like. Regardless of the apparatus and procedure used for the necking operation, typically a certain defect rate occurs during the necking operation. Accordingly, it would be useful to provide for a can fabrication procedure and apparatus which can assist in reducing the defect rate of necking operations.
Furthermore, current efforts to avoid defects during necking (and other) operations involve keeping those processes performed prior to necking under relatively strict control (including parameters such as material thickness, coating materials and temperatures and the like). Accordingly, it would be useful to provide a can fabrication procedure which results in a higher-tolerance necking operation (i.e. a necking operation which can tolerate a wider variance in parameters of upstream operations without unduly increasing necking defects), e.g. so that a greater latitude in upstream process steps can be employed without unduly increasing necking defect rates.
The present invention involves a controlled heating of the container edge prior to necking, for a desired amount of time and in such a manner to achieve a substantially circumferentially, uniform elevated temperature profile and sufficient to achieve annealing of the region to be necked. Preferably the temperature profile achieved is greatest in the region where greatest necking occurs and avoids wasting energy on heating regions of the body which will not be necked.
In one embodiment, the trimmed edge of a drawn and ironed aluminum alloy container body is heated to provide a substantially circumferentially uniform temperature profile in the region adjacent an open end, prior to necking of such region, although container bodies made of other materials could be utilized, such as steel container bodies that are intended to contain food. In particular, the temperature profile is sufficient to provide the region to be necked with an annealing temperature, preferably involving a temperature, at the trimmed edge, of (a) at least 400xc2x0 F., and more preferably (b) at least 575xc2x0 F., and most preferably (c) at least 600xc2x0 F. up to about 625F or more. Moreover, an elevated annealing temperature of at least 400xc2x0 F. at least 0.20 inches down the can side from the open end is desirable, and more desirably the annealing temperature should be 575xc2x0 F. to 600xc2x0 F. at 0.30 inches down from the open end of the can. Annealing within the above ranges is believed to improve formability during the necking operations while retaining the desired can wall strength. Moreover, a decrease in defective cans is believed to result with each elevation in temperature (a) through (c), and with an annealing temperature extending further down the can in the range of 0.20 inches to 0.40 inches.